High-quality ZnO layers grown by MBE on sapphire
Introduction
ZnO is a material which is used in many applications such as in gas sensors, transparent electrodes, and piezoelectric resonators. Moreover, it is very promising for optical applications such as in ultraviolet and blue light emitting devices because of its wide direct band gap (3.37 eV at room temperature) and high exciton binding energy of 60 meV.
Many efforts have been made to produce high-quality ZnO epilayers. In the past few years, single-crystal ZnO films have been grown using metal–organic vapor-phase epitaxy [1], pulsed-laser deposition [2], and plasma-assisted molecular beam epitaxy (MBE) [3]. MBE is recognized as a technique for preparing high-quality layers with good thickness and composition uniformity, sharp dopant profiles, growth of superlattices and heterostructures. Due to this, MBE of ZnO has been developed over the last few years [4], [5], [6], [7], with the goal of improving the material quality.
The high molecular bonding energy of O2 of 5.16 eV [8] prevents thermal dissociation of oxygen at the substrate surface. Thus, special sources of reactive oxygen species are necessary for ZnO MBE. Various types of plasma source are commonly used to produce reactive oxygen species. However, this technique has such disadvantages as contamination of growing layers by source materials [9], possible surface damage by high-energy particles, and degradation of plasma sources due to high reactivity of oxygen radicals. The above problems inspire the search for alternative sources of oxidizing species for ZnO epitaxy. The use of reactive oxygen-containing compositions, like ozone [10], as oxidants for ZnO MBE could be a very attractive alternative to the oxygen plasma cell.
Recently, we presented the implementation of H2O2 as an oxidant for MBE growth of ZnO [11]. In the present paper, we report on drastic improvements on the quality of ZnO layers grown by MBE on c-sapphire by employing hydrogen peroxide as an oxidant and MgO as a buffer layer.
Section snippets
Experimental
ZnO layers were grown on sapphire in a modified Varian Gen II MBE system using H2O2 as oxidant. The chamber was evacuated to a base pressure of 10−8 Torr. A double-zone effusion cell was used for evaporating 6N-purity elemental Zn. A H2O2 source kept in a stainless-steel vessel in a stabilized temperature water bath served as a source of reactive oxygen. A jet of vapor from the vessel was directed at the substrate through a quartz pipe connected to a leak valve. During the growth of ZnO layers,
Results and discussion
According to the RHEED, ZnO layers grown at temperatures from 350 to 650 ∘C are single crystalline. Typical streaky RHEED patterns of the ZnO layer with the electron beam incident along the and directions were observed (cf. Fig. 1). As the sample was rotated around its axis, each pattern was repeated every 60∘ which proves the hexagonal wurtzite structure of the growing film.
We used the narrowing of the layer XRD rocking curve as the optimization parameter in our investigations
Conclusion
In conclusion, ZnO epilayers were grown on (0001) sapphire substrates using hydrogen peroxide, which has proven to be an efficient oxidant for high-quality ZnO grown with MBE. The quality of the layers was improved by employing a thin MgO buffer and a subsequently grown LT ZnO buffer layer and then a main ZnO layer at a higher temperature. The excellent quality of the ZnO epiwafers obtained provides the possibility of eventually using them as an alternative to bulk ZnO wafers.
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